1. Field of the Invention
The present invention relates to dentifrice compositions containing stannous compounds.
2. Related Art
There have been many proposals in the prior art to incorporate stannous compounds into oral health care products for the purpose of achieving particular clinical benefits such as caries prevention, plaque control or the reduction of gingivitis. These stannous compounds are compounds which, upon association with water, are capable of yielding stannous ions, as it is actually the stannous ion which is active against oral bacteria.
However, the incorporation of stannous compounds into dentifrice compositions presents problems in that the stannous ions tend to react with other components of the dentifrice composition to form insoluble stannous compounds, which reduces the effective amount of stannous ions in the composition and thus renders the stannous compound less efficacious. In addition, the active stannous ion is particularly prone to oxidation, e.g. by air or an oxidizing agent, the stannous ion being converted thereby into the inactive stannic form.
In GB-A-804,486 it is proposed to overcome the problem that stannous ions react with other components of a dentifrice composition by using a slightly soluble stannous compound e.g. stannous pyrophosphate, thus maintaining a "reservoir" of stannous tin in the form of an undissociated stannous compound which replenishes stannous ions that have reacted with other components of the dentifrice composition.
We have found, however, that the inclusion of such slightly soluble stannous compounds, e.g. stannous pyrophosphate, still gives rise to the formation of inactive stannic compounds.
According to the present invention it has now been found that the conversion of stannous ions in a dentifrice composition into inactive stannic ions can be significantly reduced or prevented by the inclusion in the dentifrice composition of an antioxidant which is a radical inhibitor. Since dentifrice compositions do not normally contain an oxidizing agent and are usually packed in a closed container, it was quite unexpected that the use of an antioxidant of the radical inhibitortype did significantly reduce and prevent the conversion of stannous ions into stannic ions in such a dentifrice composition.